In both inbred and hybrid mice carrying spontaneous mammary tumor virus (MTV plus) infection, we will attempt to define the immunologic events that occur in the presence of an intact thymus or its humoral factor and their effect on the development of mammary tumors. We will study both T and B cell interaction and the role of suppressor cells. The relative role played by T and B cells in the destruction of target cells that carry tumor associated antigens, and the nature of blocking factor present in the sera of tumor bearing animals will be studied. We will attempt to determine whether or not a T cell dependent immune system is necessary or not for functional activity of serum factors, in both normal and neonatally thymectomized mice. Our immediate experiments will be directed to perform separate blocking experiments at the lymphocyte and the target cell levels in vitro, and to develop methods that will allow the measurements of antigen, antibody and complexes in the serum of tumor bearing animals. Yunis, E.J., Fernandes, G., and Greenberg, L.J. Immune Deficiency, autoimmunity, and aging. Birth Defects: Original Articles Series, Vol XI, No. 1, 185-192, 1975; Fernandes, G., Stutman, O., and Yunis, E.J. Thymic-dependent leukemia in AKR mice: Requirement of an intact thymic stroma. Amer. J. Path. 78:27a, 1975.